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Updated: June 27, 2025
But the last page in the development of these lower Radiates is but the opening chapter in that of the higher ones, and I will give some account of their transformations as they have been observed in the Acalephs.
The second class is that of Jelly-Fishes or Acalephs; and here the same plan is carried out in the form of a hemispherical gelatinous disk, the digestive cavity being hollowed, or, as it were, scooped, out of the substance of the body, which is traversed by tubes that radiate from the centre to the periphery.
The third and fourth volumes are devoted to the Radiata, and consist of five parts, namely: 1st. Acalephs in general. 2nd. Ctenophorae. 3rd. Discophorae. 4th. Hydroida. 5th. For originality of material, clearness of presentation, and beauty of illustration, these volumes have had their full recognition as models of scientific work.
In the diagram of the geological periods introduced in a previous article, I have represented all the three classes of Radiates, Polyps, Acalephs, and Echinoderms, as present on the first floor of our globe that was inhabited at all. But it is only recently that positive proofs have been found of the existence of Acalephs or Jelly-Fishes, as they are called, at that early period.
In the Hydroids, the organization does not rise above the simple digestive cavity inclosed by the double body-wall; and we might not suspect their relation to the Acalephs, did we not see the Jelly-Fish born from the Hydroid stock.
If I succeeded in explaining my subject clearly in the last article, my readers will have seen that the five Orders of the Echinoderms are but five expressions of the same idea; and I will now endeavor to show that the same identity of structural conception prevails also throughout the two other Classes of Radiates, and further, that not only the Orders within each Class, but the three Classes themselves, Echinoderms, Acalephs, and Polyps, bear the strictest comparison, founded upon close structural analysis, and are based upon one organic formula.
The most numerous of the ancient Radiates are the Acalephlan Corals, combining, in the Hydroid form, the Polyp-like mode of life, habits, and general appearance with the structure of Acalephs.
The general interest in Corals has called attention to the Polyps, and the accessible haunts of the Sea-Urchins and Star-Fishes have made the Echinoderms almost as familiar to the ordinary observer as the common sea-shells, while the Acalephs are usually to be found at a greater distance from the shore, and are not easily kept in confinement.
Beside the Jelly-Fishes arising from Hydroids, there are many others resembling these in all the essential features of their structure, but differing in their mode of development; for, although more or less Polyp-like when first born from the egg, they never become attached, nor do they ever bud or divide, but reach their mature condition without any such striking metamorphoses as those that characterize the development of the Hydroid Acalephs.
I subjoin a woodcut of a Silurian Coral, which does not, however, show the peculiar internal structure, but gives some idea of the general appearance of the old Hydroid Corals. We have but one Acalephian Coral now living, the Millepore; and it was by comparing that with these ancient ones that I first detected their relation to the Acalephs.
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